Minority carrier devices are electronic devices where performance is dominated by minority carrier behavior. There are many minority carrier devices in common use today. Examples of minority carrier devices are pn diodes laser diodes, light emitting diodes, heterojunction bipolar transistors (BHTs) and solar cells.
In operation, these devices develop large non-equilibrium concentrations of minority carriers within the device. Typically, the minority carriers must diffuse or recombine in a controlled and efficient manner in order to optimize device performance. Nearly all minority carrier devices suffer from parasitic losses due to minority carrier recombination at the surfaces of the devices. These losses can greatly reduce device performance. The quality of a surface is often characterized by the quantity "surface recombination velocity" (SRV). For most devices, high device performance is achieved with low SRV on device surfaces. In many cases where the device geometry is dominated by high surface area (as is the case with very small devices) the major performance limiting parameter is surface recombination. Unfortunately, most semiconductor materials have very high SRV on free surfaces.
Semiconductor lasers have optical and electrical parasitic losses associated with free surfaces. Defect states absorb laser light, causing nonuniform heating of the laser facet. This can limit light output, causing a catastrophic collapse of the laser device.
In the specific case of a Schottky barrier photodiode, the barrier height of many devices is typically limited by electronically active surface states between the metal-semiconductor junction.
Use of AlGaAs as a passivating layer has at least two problems. First, AlGaAs rapidly oxidizes making it unsuitable as the top window layer. Thus additional complex process steps are required at present. Second, the temperatures required in the deposition of AlGaAs are high, in some cases causing damage to the cell structure, and thus lowering the fabrication yield of the cell.
The most pervasive problem in processing such devices is the lack of a high quality passivating layer for III-V surfaces. A poorly passivated surface generates a large number of recombination centers and gives rise to parasitic losses that lower device efficiency. This ultimately limits how small a device can be made, as the ratio of exposed surface area to total device area increases as the device is scaled down. The surface of GaAs can be passivated using AlGaAs, but this is not always compatible with device fabrication techniques. Laser diodes are an example of devices that do not lend themselves to AlGaAs passivation.
For example, the degradation of III-V diode laser performance during operation has been well documented (F. A. Houle, D. L. Neiman, W. C. Tang, and H. J. Rose, J. Appl. Phys. 1992, 72, 3884). The high current and optical densities present in these structures leads to considerable heating effects due to the presence of non-radiative recombination centers. Whereas the precise mechanisms of degradation are not entirely clear, it is apparent that the creation of defects at the optical facet surface or near surface region plays a significant role in the failure of laser diodes. The facet surface is, at the onset of laser operation, an area of high surface states and, hence, non-radiative recombination centers. Localized heating combined with high electric fields, therefore, allows this surface region to exhibit a higher generation of defects which, in turn, causes a further increase in the heating effect. Diode failure or degradation is, naturally, dependent on laser design and operating parameters, but, in many cases is catastrophic in nature as a result of the self-sustained heating induced by defect states. In this respect, laser facet quality is very often the limiting factor in device performance and failure.
Therefore, a need exists for minority carrier devices which overcome or minimize the above-referenced problems.